This invention relates to the manufacture of hollow shafts having a longitudinal seam.
The manufacture of such hollow shafts generally involves forming a flat sheet of material into a generally cylindrical structure having an open seam but which is nevertheless in substantially the same shape as the desired finished shaft (referred to herein as an xe2x80x9copen shaftxe2x80x9d). The forming may be done using a series of stamping operations with curved male and female stamping dies, or the flat sheet may be fed through a series of roll forming machines which progressively bend the sheet.
The open shaft, with its open seam, is then fed from the roll forming machines to a welding machine which closes and welds the open seam to provide a closed shaft.
FIG. 1 shows a known method of closing the seam. An open shaft 10 is fed between a pair of cylindrical rollers 12, 14 which between them compress the shaft to close the seam 16. Welding occurs in a zone 18 where the opposed edges 20 of the seam meet due to the seam 16 being forced closed by the rollers 12, 14.
FIG. 2 shows the rollers of FIG. 1 in sectional elevation, along the line IIxe2x80x94II in FIG. 1. The rollers 12, 14 have axes of rotation 22 parallel to one another and perpendicular to the axis of the shaft 10. As seen in FIG. 1, the shaft 10 moves along the direction of its axis as indicated by arrow 24. The cylindrical rollers 12, 14 rotate in the directions indicated by arrows 26 and 28 (FIG. 1).
The rollers 12, 14 are immediately adjacent one another with their curved surfaces 30 close to one another. To accommodate the shaft 10, each roller has a groove 32 in its curved surface 30 defining one half of the cross section of the closed shaft. Thus, when viewed along the axis of the shaft as in FIG. 2, a substantially circular recess accommodates and forces closed the shaft to allow the opposed edges 20 to be welded to one another.
Because there is a demand for tapered shafts, it will be clear that the area of this circular recess must increase to accommodate the increasing diameter of the shaft as the shaft is fed through (assuming that the shaft is fed from the narrow end or tip to the wide end or butt). This in turn means that the groove 32 in the curved cylindrical surface 30 of each roller 12, 14 must become progressively bigger around the circumference of the cylinder. As the roller rotates when the shaft is fed through, the recess between the opposed grooves increases to accommodate the increasing taper. Thus, it can be seen in FIG. 2, that the grooves 32 at the points of the rollers 12, 14 farthest from one another are substantially bigger than at the points closest together.
In order to maintain sufficient force to close the seam of the open shaft, the rotation of the cylinders is retarded. Otherwise, the tapered shaft would simply spin the rollers to a point where the taper of the shaft is easily accommodated by the increasing recess between the grooves.
Once the shaft has been fed between the rollers, it is gripped at a pair of grip holes 34 (FIG. 1) by a pair of jaws (not shown) which are mounted on a driven carriage. The carriage is driven away from the rollers along the direction of arrow 24 to pull the shaft through the rollers. A large force must be generated to pull the shaft through the rollers to overcome the retarding force on the rollers. This force increases as the taper of the shaft increases, because the amount of force required to close a large diameter shaft is greater than that required to close a small diameter shaft. To provide sufficient structural strength at the tip of the shaft, the grip holes 34 must be spaced from the tip of the shaft by a sufficient distance x (FIG. 1) to prevent the jaws ripping through the metal between the holes 34 and the tip.
Electrical resistance welding is accomplished by applying a current along the opposed edges 20 in the zone 18, such that at the point where the edges meet, the metal melts and a join is created. The shaft must however be fed through the rollers to the point where the jaws can grip the grip holes 34 (as in FIG. 1) before continuous welding can begin. This necessarily means that there is a significant length of unusable shaft near the tip which must be cut away from the finished shaft.
The rollers 12, 14 must be made of hardened steel (or other similar hardened material, in order to generate the required closing forces consistently. They are machined from a single mass of steel and then undergo heating and cooling to achieve the correct hardness. Such rollers are very difficult to manufacture correctly due to their scale.
The hardening process requires that a very large mass of steel be machined, and then heated and cooled uniformly without cracking. One commonly experiences failure rates of 50% in manufacturing these rollers, which means that the cost of successfully manufacturing rollers is increased far above the already high cost involved in machining and tempering a steel roller of these dimensions.
The use of these rollers also has a disadvantage in that one reaches an upper limit as to the size of the roller which can be realistically made, and therefore in turn, the size of the shaft which can be made using the process of welding closed an open shaft which is closed by pressure rollers.
Yet a further disadvantage is that any particular set of rollers is confined to manufacturing a single type of pole or shaft. One cannot make a pole having different taper characteristics without designing and manufacturing a new set of rollers. Therefore the process lacks the versatility required to be able to carry out small orders, since the high cost of the rollers must be factored into the cost per shaft, and the time involved in designing and manufacturing the rollers prevents manufacturing new shapes of shaft at short notice.
The invention provides a method of welding a shaft having an open seam defined between opposed longitudinal edges, comprising the steps of closing the seam of the shaft by applying inwardly directed radial forces to the shaft and welding the seam while closed by said forces.
The invention also provides a welding apparatus comprising means for welding a shaft having an open seam defined between opposed longitudinal edges, said apparatus comprising means for applying inwardly directed radial forces to the shaft, and welding means for welding the seam while closed by said forces.
Preferably, the inwardly directed radial forces are applied at a plurality of discrete points on the shaft surface.
Preferably, the welding means comprises an electrical resistance welding arrangement.
Further, preferably, the means for applying inwardly directed radial forces to the shaft at a plurality of discrete points on the surface of the shaft comprises a plurality of pressure rollers defining an opening through which the shaft is fed, and means for varying the positions of the rollers to thereby vary the dimensions of the opening.
Optionally, a number of sets of rollers, each set defining an opening, may be provided, with the shaft passing progressively through each opening in turn.
Preferably the apparatus further comprises feeding means for moving an open shaft through said means for applying inwardly directed radial forces, whereby the open shaft is progressively closed and welded along its length.